Isotactic propylene polymers including propylene homopolymers and ethylene propylene copolymers can be produced by the polymerization of the olefin monomer units in the presence of catalysts such as Ziegler Natta catalysts or isospecific metallocene catalysts. Isotactic propylene polymers can be used in the production of molded articles in which the polypropylene is heated and then extruded through one or more dies or nozzles into a mold cavity in which it moves in both a longitudinal direction (referred to as the flow direction) and in a transverse or lateral direction (sometimes referred to as the cross flow direction). The structure of isotactic polypropylene is characterized in terms of the methyl group attached to the tertiary carbon atoms of the successive propylene monomer units lying on the same side of the main chain of the polymer. That is, the methyl groups are characterized as being all above or below the polymer chain. Isotactic polypropylene can be illustrated by the following chemical formula:
Stereoregular polymers, such as isotactic and syndiotactic polypropylene, can be characterized in terms of the Fisher projection formula. Using the Fisher projection formula, the stereochemical sequence of isotactic polypropylene as shown by Formula (1) is described as follows:
Another way of describing the structure is through the use of NMR. Bovey's NMR nomenclature for an isotactic pentad is . . . mmmmm . . . with each “m” representing a “meso” dyad, or successive methyl groups on the same side of the plane of the polymer chain. As is known in the art, any deviation or inversion in the structure of the chain lowers the degree of isotacticity and crystallinity of the polymer. In the case of random ethylene propylene copolymers, a relatively low ethylene content in the copolymer is randomly distributed throughout the polymer chain so that ethylene units are randomly interposed between the repeating propylene units.
In injection molding operations employing propylene homopolymers or copolymers, the molten polymer is introduced into a mold cavity. The molten polymer is retained in the cavity for a sufficient time to allow the desired component to form. The period of time required for the repeated cooling and subsequent removal of the molded component from the mold cavity is an important factor in the manufacturing efficiency of the injection molding operation.
During the manufacturing of a molded plastic component shrinkage within the mold and subsequent to withdrawal of the hard component from the mold results in a volume difference between the initially cast mold and the final molded article. If the dimensional changes are relatively uniform in the transverse (cross flow) and longitudinal (flow) directions of the mold, the shrinkage characteristic is considered to be isotropic. With significant differential dimensional changes in the transverse and longitudinal directions, the dimensional changes are characterized as anisotropic or differential. Warpage is caused by variations in shrinkage throughout injection molded part (D Rosata, Injection Molding Handbook, Chapman & Hall, New York, 1995). More anisotropic shrinkage often leads to warpage problems in injection molding applications. Regardless of whether the shrinkage characterized is isotropic or anisotropic, the relative shrinkage should be taken into account in order to obtain the end use articles of the molded article of the correct dimension.